1. Field of the Invention
The present invention relates to an optical module for light path conversion which is used in the fields of optical communication, optical information processing and the like.
2. Description of the Related Art
In recent years, as the practical adaptation of optical communications systems and optical information processing systems has progressed, a demand has arisen for systems that have an even higher degree of function. Optical integrated circuits in which optical functional elements are integrated are indispensable for the realization of such systems, and such circuits have been widely investigated.
Especially in regard to techniques for connecting optical semiconductor elements with optical fibers, the active alignment method has commonly been used in the past for optical connections between optical semiconductor elements and optical fibers. This active alignment method is a method in which optical connections between optical fibers and optical semiconductor elements are made by causing the element to emit light or causing light to be incident on one end of the element waveguide, then disposing the optical fiber at the emitting end, and finely adjusting the position of the optical fiber so that the quantity of light received by the optical fiber reaches a maximum.
In this active alignment method, it is necessary to cause the element itself to emit light, or to cause light to be incident from one end of the element waveguide. In this method furthermore, time is required in order to center the optical axis for each individual element involved; accordingly, this method suffers from inconveniences such as increased cost and the like.
In order to solve this problem, passive alignment techniques have been actively investigated in recent years. Such passive alignment techniques are techniques in which optical connections are achieved by mechanically adjusting the relative positions of the optical semiconductor element with respect to optical fibers at high precision. In the case of such techniques, the positions of the optical semiconductor element and optical fiber are determined only with mechanical precision; accordingly, there is no need to cause the optical semiconductor element to emit light, or to caused light to be incident on the optical semiconductor element. Thus, passive alignment techniques may be said to be an extension of conventional electrical element mounting techniques; the effect in terms of mass production is extremely great, and such techniques are becoming essential in order to lower the cost of optical modules. Furthermore, passive alignment techniques are indispensable for realizing surface-mounted type optical transmission modules, for increasing the speed of optical modules, and for reducing the size and height of optical modules.
In recent years, meanwhile, the application and development of surface emitting lasers (vertical cavity surface emitting lasers; hereafter abbreviated to xe2x80x9cVCSELxe2x80x9d) have been widely discussed. Compared to conventional Fabry-Perot lasers which are end-surface resonator type lasers, such VCSEL have superior special features such as a small operating current and superior temperature characteristics; accordingly, these lasers have attracted attention as the next generation of light sources for use in optical communications.
In consideration of such trends, it is very natural that efforts are now being made to apply passive alignment techniques to VCSELs.
However, three major problems have been encountered in realizing optical modules with surface-mounted VCSEL.
The first problem is that since the mounting surface of a VCSEL and the direction of light emission of a VCSEL are in a mutually perpendicular relationship, special devices must be used when a VCSEL and optical fiber are optically connected. Since a VCSEL is a surface-mounted type device, the direction of light emission is the normal direction with respect to the element substrate. Since the electrodes that supply the current are generally formed on the surface of the substrate, light rays are emitted in the normal direction with respect to the surface of the mounting substrate is the electrodes are connected to the electrical connection parts on the surface of the mounting substrate. Specifically, the direction of travel of the light rays and the mounting surface are in a perpendicular positional relationship. In conventional end-surface emitting lasers, there is no need to give any consideration to such a problem. Ordinarily, the resonator is formed parallel to the mounting surface, so that the direction of light emission in a conventional end-surface emitting laser is the direction of the resonator. As a result, the emitted light is emitted parallel to the mounting surface. Accordingly, the emitted light from the laser diode can easily be caused to be incident on an optical fiber by positioning the optical fiber on one end surface of the resonator.
As a result of the above facts, a method in which (for example) an inclined reflective face is formed on the surface layer of the mounting substrate, and the light path of the emitted light is bent by this reflective face so that the emitted light is caused to be incident on an optical fiber disposed in a predetermined position, or a device known as a chip carrier which realizes an optical connection by mounting the VCSEL on one surface of a rectangular-solid block, and causing the incident end surface of the optical fiber and the emitting end surface of the VCSEL to face each other, must be used in order to cause the emitted light from the VCSEL to be incident on an optical fiber.
The second problem is that there are no effective means for constantly monitoring the optical output power of a VCSEL. In semiconductor lasers, as represented by VCSEL, the operating current value of the laser and the differential light emission efficiency fluctuate according to the ambient temperature environment and the like. The differential light emission efficiency is the ratio of the intensity of the optical output power to the injected current. Compared to conventional end-surface emission type elements, a VCSEL has the special features of stability with respect to temperature and superior long-term reliability; in some cases, however, monitoring of the intensity of the emitted light is necessary because of fluctuations in the external environment, deterioration over time or the like.
A method in which (for example) the intensity of the emitted light is monitored by installing a photodiode between the emitting end surface of the VCSEL and the incident end surface of the optical fiber, or by installing a half-mirror between these two end surfaces, is immediately conceivable as a method of solving the two problems mentioned above, constantly monitoring the intensity of the emitted light of the VCSEL, and realizing a surface-mounted type optical module. However, since the gap between these two end surfaces is approximately several microns to several hundred microns, such a method is not realistic.
Meanwhile, an example of a conventional optical module is disclosed in Japanese Patent Application Laid-Open No. 2000-502819 as a proposal regarding the two problems mentioned above. This example is a method in which a VCSEL and a photodiode are disposed on the surface of a mounting substrate so that the light emitting end surfaces or light receiving end surfaces face in the normal direction of the mounting substrate, a reflective element which has an optical coupling element that focuses the emitted light and causes this light to be incident on an optical fiber is provided, and the intensity of the emitted light of the VCSEL is monitored by causing a portion of the emitted light to be reflected by the reflective element, and detecting this light with the photodiode.
In the case of this method, however, constituent parts such as a focusing element, reflective element or the like are required; accordingly, not only is the cost increased, but a coaxial type optical module must be formed in order to realize this structure, so that the realization of a surface-mounted type optical module using a VCSEL is difficult.
Furthermore, an example of a conventional optical module is disclosed in U.S. Pat. No. 6,081,638. This example is a surface-mounted type system in which light path conversion is accomplished by reflecting the emitted light from the VCSEL by means of the tip end surface of an optical fiber worked into an inclined shape, or a semi-transparent reflective surface, and simultaneously receiving a portion of the transmitted light by means of a photodiode.
In the case of this system, however, it is necessary to polish the end surface of the optical fiber into an inclined surface; accordingly, the cost is increased. Furthermore, there are other practical problems such as the need to adjust the direction of rotation of the optical fiber axis in order to eliminate cylindrical symmetry of the optical fiber.
The third problem is that the mounting precision of a VCSEL is poor. In an end-surface emission type laser module, the mounting substrate and the surface of the active layer of the laser diode are caused to face each other, and the relative positions of both parts are determined by means of alignment marks formed on the respective surfaces. A high-precision V groove is formed beforehand in the surface layer of the mounting substrate using a silicon anisotropic etching technique or the like, and high-precision positioning of the laser diode and optical fiber is realized by disposing the optical fiber on this V groove.
However, in the case of a VCSEL, the mounting surface and the direction of the emitted light are perpendicular. As a result, if mounting is performed with the mounting substrate and the light emitting point of the VCSEL facing each other, the path of the emitted light is blocked. Accordingly, the abovementioned mounting method is not suitable.
In light of the above facts, a method in which the back surface of the VCSEL is caused to face the mounting substrate, and positioning is performed using the back surface electrodes or back surface element shape of the VCSEL as a reference might also be conceived; however, such positioning cannot be accurately performed with respect to the position of the light emitting point of the VCSEL, and this has a great effect on the mounting precision. This third problem is also encountered in the abovementioned U.S. Pat. No. 6,081,638 relating to a surface-mounted type optical module using a VCSEL.
Thus, in the past, it has been impossible to achieve a simultaneous solution of the three problems mentioned above, regardless of the technique used.
It is an object of the present invention to provide an optical module which simultaneously solves the three problems mentioned above, and which also has a simple structure.
This object is achieved by means of an optical module which comprises a surface light emitting element and a light path converting reflective body that has an inclined face on which a light-reflecting film is formed, and that consists of single-crystal silicon, wherein the light that is emitted from the surface light emitting element is subjected to light path conversion by the inclined face.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments/examples with reference to the accompanying drawings.